The present embodiments relate to a balanced-to-unbalanced transformer for converting a symmetrical high-frequency signal into an asymmetrical high-frequency signal.
Balanced-to-unbalanced transformers are symmetrical arrangements (e.g., baluns) that are employed in a plurality of technical application areas. In this role, one task of a balun is to suppress unwanted common-mode excitation occurring during the conversion of the symmetrical high-frequency signal into the asymmetrical high-frequency signal to the greatest extent possible in order to avoid, for example, standing waves during a transmission over a coaxial cable. In the field of magnetic resonance tomography, baluns are employed without the use of ferrites (e.g., in receiving devices for high-frequency signals). Information about the tissue examined using the tomography system is derivable from the high-frequency signals. High-frequency signals in the 500 MHz range and below may be processed in magnetic resonance tomography systems. The large wavelengths resulting herefrom impose large demands in terms of space requirements and consequently on the feasibility of employing baluns in discrete circuits.
A balanced-to-unbalanced transformer may be implemented in the form of a two-wire line, in which two coil windings electrically insulated from each other are wound side by side on a winding form. Due to the inductance of the individual windings and a parasitic capacitance resulting on account of the spacing between turns of the individual winding, a resonant circuit is formed. The resonant frequency of the resonant circuit may correspond to a frequency of the high-frequency signal that is to be converted. In such an arrangement, the two conductors of the coil windings for the differential-mode excitation act as a parallel wire system that, except for ohmic conductor losses, has no effect on the transmission of the differential-mode signal. For the common-mode excitation at the resonant frequency, in contrast, the balun acts as a parallel resonant circuit. A finite quality of the parallel resonant circuit determines the degree of common-mode suppression.